Signal processing circuits are frequently used to read storage media and interpret obtained analog signals as discrete values stored on the media. For magnetic storage media, a transducer head may fly on a cushion of air over a magnetic disk surface. The transducer converts magnetic field variations into an analog electrical signal. The analog signal is amplified, converted to a digital signal and interpreted (e.g., using maximum likelihood techniques, such as using a Viterbi detector). Tracking of stored data during a read operation is frequently performed using feedback or decision aided gain and timing control. Moreover, a closed-loop servo system, driven by servo information embedded in a dedicated portion of every track of the recording surface, is used to accurately position the head and follow the track.
The servo control information defines the position of the data tracks. Typically, the servo control information is written on each surface as a radially extending set of spokes or wedges. The servo information typically includes a servo index mark (SIM) used as a reference point for counting sectors in a track. The servo information also typically includes, in a servo wedge at a track, a preamble used for timing recovery and data framing, a servo address mark (SAM) used for detecting the beginning of a valid servo field, and a track number code associated with the current track (and current wedge). The servo control information can also include two or more fine-positioned offset bursts configured in an echelon across the track.